Recent developments in the art of manufacturing has brought significant reduction in cost and form factor of mobile consumer devices—tablet, blue tooth headset, net book, net TV etc. As a result, there is an explosive growth in consumption of these consumer devices. Besides communication applications such as voice and video telephony, voice driven machine applications are becoming increasing popular as well. Voice based machine applications include voice driven automated attendants, command recognition, speech recognition, voice based search engine, networked games and such. Video conferencing and other display oriented applications require the user to watch the screen from a hand-held distance. In the hand-held mode, the signal to noise ratio of the desired voice signal at the microphone is severely degraded, both due to the exposure to ambient noise and the exposure to loud acoustic echo feedback from the loudspeakers in close proximity. This is further exacerbated by the fact that voice driven applications and improved voice communications require wide band voice.
A few examples of the devices which benefit from this invention are shown in FIG. 1. These examples include audio hosts 010 and audio accessory 011 headset. They typically contain a microphone 013. The look direction of the targeted voice source 014, is typically known a priori as depicted. The interfering noise sources, henceforth collectively called ambient noise 015, arrive from directions other than the look direction. For the purposes of describing the current invention, the acoustic echo 016 generated by the loudspeakers 019 shall also be treated as ambient noise. The loudspeakers 019 are placed such that the echo arrives from a direction which is generally orthogonal to the said look direction.
The said voice sensing problem due to the reduced signal to noise ratio can be addressed by employing multiple microphones. As shown in FIG. 2, some recent devices have started introducing a second microphone, i.e. 2 MIC array 021, which forms either an end-fire or a broadside beam in the desired look direction. These rudimentary beam forming solutions have several disadvantages. For instance, they introduce frequency distortion, since the beam angular response is frequency dependant.
An alternate method called blind source separation (BSS) has been discussed in the academia. Given two microphones placed in strategic locations with respect to two sources of sound, it is possible to separate out the two sources without any distortion. As shown in FIG. 3, the first microphone 031 is placed close to the first sound source 032, capturing a first sound mixture 033 predominated by the first sound source. Similarly the second microphone 034 is placed in the proximity of the second source 305, generating a sound mixture 036 predominated by the second source. The source separation unit 037 generates two outputs 038, separating the two sound sources with little or no distortion. However, in the real world, it is not practical to place a microphone close to the ambient noise, but away from the target voice.
It is within this context that the embodiments arise.